Solar cells are used to convert sunlight into electricity using a photovoltaic effect. A general object is to achieve high conversion efficiency balanced by a need for low production costs.
In order to obtain high efficiency, semiconductor materials of good quality should be used as substrates for the solar cell and the surfaces of the substrates should be highly passivated in order to minimize any recombination losses. Furthermore, contact schemes for electrically contacting the substrate should be optimized in order to minimize resistance losses and shading.
In order to keep production costs low, it is generally intended to use as few processing steps as possible and furthermore to prevent complicated and costs-intensive production steps such as e.g. photolithography masking and high temperature processing steps.
A solar cell concept has been proposed in WO 03/083955 A1. Therein, a rear-junction type photovoltaic element is proposed in which a pn junction and electrodes are formed on a rear surface opposite to a light incident surface of a semiconductor silicon substrate. The photovoltaic element has an intrinsic semiconductor film on its rear side having a thickness ranging from 0.1 nm to 50 nm. On the back side of the intrinsic semiconductor film, p-type conductive semiconductor portions and n-type conductive semiconductor portions are disposed and each of the conductive semiconductor portions is contacted with a respective first or second electrode. Such heterojunction intrinsic-thin film solar cells is sometimes referred to as HIT solar cell.
Several attempts to improve this solar cell concept and methods for fabricating such solar cells have been proposed. Therein, the fabricating methods comprise several processing steps inter alia including different approaches for passivating the substrate surface, defining the geometry of the various semiconductor electrode layers and insulator layers and/or applying electrical electrodes to the solar cell substrate. For example, it has been proposed to use expensive photolithography processing steps to precisely define areas of semiconductor layers or insulator layers on the substrate's rear surface. Furthermore, it has been proposed to locally remove previously deposited layers by locally etching in order to subsequently deposit other layer materials in the prepared openings thereby generating a desired geometry of a rear side layer arrangement.
However, it seems that all conventional approaches for generating a rear contact heterojunction intrinsic thin film solar cell scheme suffer from at least one of the following shortcomings:                a lack of a cheap mass-production method        poor passivation at electrode edges due to imperfect definition of electrode areas        a risk of shunting forcing metallization only over a limited part of the electrodes        difficulties in controlling the cleanness and deposition homogeneity of the intrinsic thin film in the critical layer closest to the substrate        expensive layer geometry definition due to the use of lithography        a requirement of using <100>-orientated silicon wafers as substrates.        